1. Field of the Invention
The present invention relates to a light responsive semiconductor switch with shorted load protection for use in an optical relay.
2. Description of the Prior Art
Japanese Patent Publication No. 11-163706 discloses a light responsive semiconductor switch for use in an optical relay. The switch includes a photovoltaic element which provides an operating voltage upon absorption of light from a light source, and an output transistor which is triggered by the operating voltage to become conductive for connecting a load to a power source. In order to protect the output transistor from an overcurrent due to an accidental short-circuiting of the load, the switch includes an overcurrent sensor for detection of the overcurrent condition, and a shunt transistor which, in response to the overcurrent condition, becomes conductive to flow the current from the photovoltaic element away from the output transistor to turn if off for interruption of the overcurrent. Further, the switch includes a latch circuit which, in response to the overcurrent condition, provides and holds an interruption signal fed to a control electrode of the output transistor to keep turning off the output transistor for continued interruption of the overcurrent. In this prior art, the shunt transistor is included in the latch circuit to be responsible also for the latching operation. Therefore, the shunt transistor has to satisfy two different requirements, one for the turning off of the output transistor, and the other for holding the interruption signal applied to the control electrode of the output transistor in association with a resistor in the latch circuit. With this restriction to the shunt transistor common to the latch circuit, it is rather difficult to combine the two requirements against the use of the photovoltaic element of varying current generating capacity. For example, when the photovoltaic element having a large current generating capacity is used to apply a correspondingly high voltage to the control electrode of the output transistor for rapidly turning it on, the conduction of the shunt transistor made for the latching operation may not be enough to lower the voltage applied to the control electrode of the output transistor below a threshold voltage thereof, failing to turn off the output transistor even when the shunt transistor is made conductive to draw the current from the photovoltaic element. Accordingly, the prior switch poses limitations to a circuit design and is not satisfactory for complete interruption of the overcurrent irrespective of the current generating capacity of the photovoltaic element.
In view of the above insufficiency in the prior art, the present invention has been achieved to provide an improved light responsive semiconductor switch with shorted load protection which is capable of successfully interrupting a load overcurrent. The semiconductor switch in accordance with the present invention comprises an output switching transistor connected between a pair of output terminals which are adapted for connection to a load circuit composed of a load and a power source energizing the load. The output switching transistor has a control electrode with a threshold voltage at which the output switching transistor conducts to connect the load to the power source. A photovoltaic element is included in the switch to generate an electric power upon absorption of light from a light source. The electric power provides an operating voltage decreasing with an increasing current flowing from the photovoltaic element. An overcurrent sensor is coupled to the load circuit to provide an overcurrent signal when the load circuit sees an overcurrent flowing through the load from the power source. A shunt transistor is connected in series with a current limiting resistive element across the photovoltaic element to define a shunt path of flowing the current from the photovoltaic element through the current limiting resistive element away from the output switching transistor. Also included in the switch is a latch circuit which is connected to the overcurrent sensor and the shunt transistor. The latch circuit is energized by the photovoltaic element and provides an interruption signal once the overcurrent signal is received and holds the interruption signal after the removal of the overcurrent signal. The interruption signal causes the shunt transistor to become conductive to flow the current from the photovoltaic element through the shunt path, lowering the operating voltage being applied to the control electrode of the output switching transistor below the threshold voltage so as to turn off the output switching transistor for disconnection of the load from the power source.
The characterizing feature of the present invention resides in that the shunt transistor and the current limiting resistive element are formed separately from the latch circuit, and that the current limiting resistive element is connected between the control electrode of the output switching transistor and the positive electrode of the photovoltaic element so as to limit the current from the photovoltaic element, when said shunt transistor is conductive, to such an extent as to lower the operating voltage being applied to the control electrode of the output switching transistor below the threshold voltage, while allowing the photovoltaic element to provide a supply voltage to the latch circuit for holding the interruption signal. Thus, the series combination of the current limiting resistive element and the shunt transistor which are separately formed from the latch circuit can assure to provide the supply voltage to the latch circuit and at the same time to limit the operative voltage being applied to the control electrode of the output switching transistor, so as to keep the interruption signal from the latch circuit on one hand, and to turn off the output switching transistor without fail in response to the interruption signal on the other hand, enabling successful and reliable interruption of the overcurrent. Also, since the current limiting resistive element is separately formed from the latch circuit, it is readily possible to assure the above interruption of the overcurrent irrespective of varying current generating capacity of the photovoltaic element, simply by selecting the impedance of the current limiting resistive element. With this result, the output transistor can be protected completely from the overcurrent in the load circuit.
In one version of the present invention, the overcurrent sensor is realized by a current sensing resistor inserted in the load circuit, and a transistor switch which is disposed to receive a voltage developed across the current sensing resistor to provide the overcurrent signal to the latch circuit when the voltage exceeds a predetermined level.
In another version of the present invention, the overcurrent sensor is realized by a current sensing resistor connected in series with a bypass switching transistor between the output terminals and in parallel with the output switching transistor, and a transistor switch which is disposed to receive a voltage developed across the current sensing resistor to provide the overcurrent signal to the latch circuit when the voltage exceeds a predetermined level.
For driving the load energized the DC power supply, the output switching transistor is preferably defined by a single metal oxide semiconductor field-effect transistor (MOSFET) whose gate-source is connected across the photovoltaic element, and whose drain-source is connected between the output terminals.
For driving the load energized by the AC power supply, the switch is preferred to include a pair of output switching transistors each in the form of a metal oxide semiconductor field-effect transistor (MOSFET). The two output switching transistors are connected in series between the output terminals with sources of the individual MOSFETs being connected to each other and with gates of the individual MOSFETs being commonly connected to receive the operating voltage from the photovoltaic element.
Preferably, the latch circuit is realized by a flip flop having a set input, a reset input, and an output. The set input is connected to receive the overcurrent signal and the reset input is connected to receive the operating voltage from the photovoltaic element, while the output is connected to turn on and off the shunt transistor.
The shunt transistor is preferred to be a metal oxide semiconductor field-effect transistor (MOSFET) whose drain-source is connected in series with the current limiting resistive element across the photovoltaic element. In this connection, the flip-flop is realized by a combination of a first resistive element and a first metal oxide semiconductor field-effect transistor (MOSFET) whose drain-source is connected in series with the first resistive element across the photovoltaic element, and a combination of a second resistive element and a second metal oxide semiconductor field-effect transistor (MOSFET) whose drain-source is connected in series with the second resistive element across the photovoltaic element. The first MOSFET has a gate connected to a point between the second resistive element and a drain of the second MOSFET. The second MOSFET has a gate connected to a point between the first resistive element and a drain of the first MOSFET. The point between the second resistive element and the drain of the second MOSFET is also connected to the gate of the shunt transistor (MOSFET) so as to provide the interruption signal to the gate of the shunt transistor. The second MOSFET receives at its gate the operating voltage from the photovoltaic element through the first resistive element so as to become conductive, thereby lowering the operating voltage applied through the second resistive element to the gate of the first MOSFET and to the gate of the shunt transistor (MOSFET) to make the first MOSFET and the shunt transistor (MOSFET) non-conductive, thereby applying the operating voltage to the control electrode of the output switching transistor to turn it on. The second MOSFET also receives at its gate the overcurrent signal which makes the second MOSFET nonconductive, thereby raising the voltage applied to the gates of the first MOSFET and the shunt MOSFET so as to make the first MOSFET and the shunt transistor (MOSFET) conductive, which keeps the second MOSFET non-conductive for continued conduction of the shunt transistor (MOSFET) for keeping the interruption of the output switching transistor until removal of the operating voltage from the photovoltaic element.
For the above circuit configuration, each of the current limiting resistive element, the first resistive element and the second resistive element is preferably in the form of a punch-through space charge resistor. The punch-through space charge resistor is realized by a semiconductor substrate having a conductive type which is one of n-type and p-type, a well diffused in the surface of the substrate and being of a conductive type opposite of the substrate, and a pair of regions diffused in the surface of the well in a spaced relation with each other. The regions are of the same conductive type as the substrate. Electrodes are respectively formed on the regions to apply the operating voltage between the regions partly through the well. In this condition, the regions are cooperative to form therebetween a depletion layer responsible for carrying a minute current and therefore defining resistance for each of the current limiting resistive element, the first resistive element and the second resistive element. Since the punch-through space charge resistor can realized into a micro structure while exhibiting a high resistance, the whole switch can be made compact even when the photovoltaic element of small current generating capacity is utilized to require a considerably high resistance for each resistive element.
Alternatively, the current limiting resistive element, the first resistive element and the second resistive element may be realized by diodes, respectively.
Preferably, the output switching transistor is realized by a metal oxide semiconductor field-effect transistor (MOSFET) having a gate defining the control electrode. A zener diode is connected across gate-source of the output switching transistor in parallel with the photovoltaic element in such a manner as to connect a cathode of zener diode to the gate of the output switching transistor. The zener diode is selected to have a breakdown voltage higher than an open-circuit voltage of the photovoltaic element. Thus, even when an excessively large voltage is applied to the output switching transistor due to the load short circuit, the zener diode can clamp the gate voltage of the output switching transistor to the breakdown voltage so as to protect the output transistor from destructive voltage.
A diode may be being connected across the current limiting resistive element with an anode of the diode connected to the gate of the output switching transistor. Thus, when the photovoltaic elements is turned off to cease providing the operating voltage, the diode establishes a bypass across the current limiting resistive element for discharging the charges accumulated in the gate of the output switching transistor, thereby speeding up the discharge to rapidly turn off the output switching transistor.
In this connection, a resistor may be connected in series with the diode across the current limiting resistive element in order to avoid malfunction of the short-circuit interruption of the switch when the load circuit is subjected to a high voltage noise such as a lightning surge. Upon occurrence of the high voltage noise while the output switching transistor is kept turned on, a rushing current would flow from the drain to the gate of the output switching transistor (MOSFET) through a parasitic capacitance in the drain-gate path into the photovoltaic element, thereby instantaneously canceling the operating voltage of the photovoltaic element. If this should occur, the latch circuit would be reset to turn off the shunt transistor, disabling the interruption of the overcurrent in the load circuit, failing to protect the load circuit as well as the output switching transistor. However, the resistor included in the above bypass can well delimit the rushing current to avoid the unintended reset of the latch circuit and assure a safe protection of interrupting the overcurrent against the high voltage noise.
Instead of the diode connected across the current limiting resistive element, a discharging metal oxide semiconductor field-effect transistor (MOSFET) may be utilized for the same purpose of rapidly turning off the output switching transistor in response to the deactivation of the photovoltaic element. The discharging MOSFET has a source coupled to a connection between the current limiting resistor element and the positive electrode of the photovoltaic element and has a drain and a gate which are commonly connected to the gate of the output switching transistor for discharging the charge accumulated in the gate of the output switching transistor through the MOSFET when the photovoltaic element is deactivated.
Preferably, the transistor forming the overcurrent sensor is realized by a third metal oxide semiconductor field-effect transistor (MOSFET) which provides the overcurrent signal to the latch circuit upon being turned on. In this connection, an additional photovoltaic element may be used to provide an offset voltage, upon absorption of the light, which is added to a detected voltage appearing across the current sensing resistor. The additional photovoltaic element is connected in circuit to the third MOSFET so as to turn on the third MOSFET when the detected voltage plus the offset voltage exceed a predetermined level. Thus, even a relatively low detection voltage across the current sensing resistor can successfully trigger the third MOSFET for increasing sensitivity of the third MOSFET or the overcurrent sensor to the overcurrent condition. Therefore, the third MOSFET can well respond to a low level overcurrent for successfully protecting the output switching transistor therefrom.
In another version of the present invention, the switch further includes a second shunt transistor in the form of a metal oxide semiconductor field-effect transistor (MOSFET) connected across the shunt transistor. The second shunt transistor has a drain which is connected to a point between the control electrode of the output switching transistor and the current limiting resistive element, and has a source which is connected to the source of the shunt transistor. The second shunt transistor has a gate which is connected to directly receive the voltage developed across the current sensing resistor such that, in response to the voltage of the current sensing resistor exceeding the predetermined level, the second shunt transistor becomes conductive to flow the current from the photovoltaic element through the current limiting resistive element and through the second shunt transistor away from the output switching transistor prior to the latch circuit responding to provide the interruption signal of turning on the shunt transistor. With this arrangement, the output switching transistor can be turned off for interruption of the overcurrent in prompt response to the overcurrent condition without having to wait for the actuation of the latch circuit, and can be held turned off by the subsequently actuated latch circuit. Thus, it is readily possible to give a more reliable protection of the output switching transistor even against an instantaneous overcurrent flow.
The switch may include a biasing means for supplying a bias current from the photovoltaic element to the gate of the shunt transistor when the latch circuit provides the interruption signal in response to the overcurrent signal. With the addition of the bias current or the bias voltage to the gate of the shunt transistor, the shunt transistor can be promptly triggered to turn on for rapid interruption of the overcurrent through the output switching transistor, thereby protecting the output switching transistor as well as Fe associated element effectively.
Also, the switch may include a block circuit for blocking the current of the photovoltaic element from flowing to the gate of the output switching transistor when the latch circuit provides the interruption signal in response to the overcurrent signal. With the inclusion of the blocking circuit, the current from the photovoltaic element is intensively utilized for triggering the shunt transistor, thereby quickening the interruption of the overcurrent for immediate protection of the output switching transistor against the overcurrent.
Further, the overcurrent sensor may include a low-pass filter which negates or cancel a high frequency voltage appearing across the current sensing resistor so that the overcurrent sensor provides the overcurrent signal to the latch circuit only when the voltage across the current sensing resistor exceeds the predetermined level and lasts over a certain time period. Thus, the latch circuit is prevented from providing the interruption signal in response to noncritical overcurrent appearing only instantaneously in the load circuit such as minor noises and rushing current which does not require the protection of the output switching transistor.
Moreover, the switch may include a delay timer which delays providing the overcurrent signal from the overcurrent sensor to the latch circuit for a short time period immediately upon the photovoltaic element generating the electric power, thereby canceling a transient voltage appearing across the current sensing resistor immediately after the activation of the photovoltaic element. With the inclusion of the delay timer, the output switching transistor can be prevented from responding to noncritical overcurrent appearing immediately after the actuation of the photovoltaic element for assuring reliable switching operation.
These and still other objects and advantageous features of the present invention will become more apparent from the following description of the embodiments when taken in conjunction with the attached drawings.